Purifying zinc vapor



Mlli'h 12, l935- L. s. H'oLs'TElN Er AL 1,994,345

I PURIFYING vZINC VAPOR Filed May 28, 1951 ATTORN EYJ Patented Mart l? 'rara FFE .orientation may as, resi, seriali No. masts! 2@ @tarima of very high purity from relatively impure zincl metal. l

Metallic zinc has heretofore been commonly re-` distilled for the purpose of freeing it from lead and iron. In these heretofore customary processes ,pf redistillation, the removal of lead, in particular, has not been accomplished to the extent that would be expected from the vapor-pressure of lead at the boiling point of zinc.

The decrease in the vapor tension of lead produced by its solution in molten zinc does not, according to experience, decrease correspondingly the amount of lead actually distilled over in the ordinary commercial practice of redistillation. In such an operation, when carried out at an adequate dstilling rate to render the operation proiitable, molten zinc carrying lead in solution splashes up against the superheated sides of the distilling retort above the bath of molten metal; the zinc promptly boils olf, leaving a nlm consisting substantially of lead, which tends to establish the true vapor-tension of lead in the vapor in the retort. The difference in the amount of lead actually present in redistilled zinc and the amount which theoretically would be expected is,- We believe. largely due to the following factors:

1. Superheating of eine capot-In the ordinary practice of zinc redistillation, the vapor is in contact with and exposed to radiation from the externally heated walls of the retort; or, in the case of internally heated retorts, the vapor is in contact with, and exposed to radiation from, the electric resistor or arc or other means used to generate the heat. The zinc vapor is thus heated to a temperature exceeding the boiling point of zinc. At this temperature' the zinc vapor will absorb in vapor form a greater percentage of lead by volume than corresponds to the vapor tension (or partial pressure) of lead at thesboiling point of zinc. This excess lead vapor iscarried over into the condenser by one or both of the following phenomena, even in cases Where the zinc vapor loses its superheat before entering the condenser or the vapor conduit leading thereto.

2. Supersaturation.-Lead that has been volatllized by superheating as described above will boiling zinc.

not condense out of the zinc vapor at once when 1 the mixture of zinc Vapor and lead vapor has been cooled to the boiling point of zinc. The excess lead (that is in excess of the amount that would theoretically be present at the existing partial 5 pressure) has a tendency to remain in vapor form for a certainperiod of time without condensing into molten lead, and this delay gives sufiicient time for the lead to be carried into the condenser by the zinc vapor.

3.,Entranment.-Droplets of molten lead or molten zinc mixed with molten lead are formed in the current of zinc vapor either by the condensation of the supersaturated lead vapor or by the mechanical spraying of droplets of molten zinc contaminated with lead into the vaporY streamV by bursting bubbles of zinc vapor in the bath of These droplets of molten 'metal contaminated with lead may be carried mechani cally (entrained) into the zinc condenser in the form of a metallic mist by the zinc vapor.

Lead is herein used as a specific example of the manner in which these three factors interfere with the purification of zinc by redistillation. It should be understood that other metals with boiling points higher than that of zinc behave in the same manner and, therefore, are not eliminated from zinc by redistillation to the extent that would be expected from their low vapor tensions at the boiling point of zinc.

TheV present invention is based upon a careful consideration of 'these factors and involves an improved method of purifying zinc vapor in which the deleterious inuences of these three factors are eectlvely avoided, as a result of which zinc metal of high purity may be obtained. While the invention is especially adapted to the purica.- tion of zinc vapor by redistillation, it is equally applicable in principle to the purcation of zinc vapor from any source, such, for example, as a 40 zinc smelting or reduction operationin which the zinc vapor is admixed with carbon monoxide gas. In its broad aspect, the invention takes advantage of reflux condensing to provide an eliminating or purifying agency through or in contact withwhich the relatively impure zinc/vapor passes. Thus a part of the zinc vapor undergoing purication is condensed to molten zinc, as, for example, in a fractionating or refluxing column and this condensed molten zinc passes back to the volatilizing source of the zinc vapor in countercurrent flow to the stream of zinc vapor. In effect, the condensed molten zinc exercises a scouring or scrubbing eect on the relatively impure zinc vapor, and collects alarge proportion of the 55 lead, iron and other metals of higher boiling points than zinc and conveys these metal impurities out of the stream of zinc vapor.

The characteristic feature of the invention is the reflux condensing action as a consequence of which the metal impurities are largely removed from the zinc vapor. Incarrying out the invention for purifying zinc vapor derived from a redistillation retort, a portion of the zinc vapor derived from the retort is condensed in the fractionating column or reflux condenser and returned to the retort (reuxed) in such a manner that all the zinc vapor leaving the retort is brought in intimate contact with the returning stream (or perhaps shower or mist) of molten zinc. This condensation and return of molten zinc to the distilling retort (or other volatilizing source of zinc vapor) is brought about by conductingv the zinc vapor from the retort into an upright (preferably substantially vertical) conduit so constructed and positioned that there is a regulable heat loss or dissipation from its outer surface (by radiation, conduction and convection). The amount of heat dissipated from this upright conduit may be regulated by varying the area of its outer sur-l face (that is the surface through which the heat contained in it must pass to be dissipated) by surrounding it with heat insulation of regulable thickness, and by shielding it, if necessary, from the heat from the furnace that heats the distillation retort. This upwardly extending conduit constitutes the reflux condenser or fractionating column. Sufcient heat is dissipated from it to recondense part of the zinc vapor evolved from the volatilizving source. In practice we have found that effective removal of lead (and other contaminants with high boiling points) from une zinc vapor is very satisfactorily carried out if the reflux condenser be so operated as to condense and return to the zinc distillation retort from 15 to 20 per cent. of the zinc vapor evolved therefrom. The zinc vapor not recondensed by the reflux condenser enters an appropriate conduit leading to any suitable type of zincvapor treatment device; such as a condenser capable of eiciently condensing large quantities of zinc Vapor.

It is advantageous to fill the reflux condenser or fractionating tower with packing material with the object of rendering Vthe flow of both the zinc vapor and the recondensed molten zinc on its Way back to the distillng retort tortuous, thereby bringing about intimate and relatively prolonged contact between the molten zinc andthe zinc vapor as they move in counter-current paths. The filling or packing material permits a shorter length of column, and where omitted the reflux condenser or fractionating column must be vof sufficient length or height to bring about sufficient contact between the ascending zinc vapor and the refluxed molten zinc to effect the desired result. The filling or packing material may consist of any substance vthat is unaffected by zinc vapor and molten zinc.

bide, clay, and the like, crushed brick and the like may be used. The packing material should be crushed to such a size as to leave adequate voids for the passage of the zinc vapor and molten zinc metal without clogging and creating excessive back pressure. Satisfactory materials used in practice have been coke, crushed to about 3A", and a mixture of `crushed refractory brick and reclay, crushed to about It is advantageous to use material that crushes with a conture.

Thus, lumps of i coke, graphite, refractories such as silicon carchoidal fracture (such as coke ,or fiint clay), since such lumps pile up with point and line contacts instead of surface to surface contacts so that adequate porosity is produced.

Any appropriate form of packing material such as spheres, grids or Raschig rings may, of course, be used. Furthermore, the reux condenser` or fractionating column may itself consist of spiral or tortuous passages or may be equipped with baffles to produce the desired effect.

The stream of molten zinc descending in the reflux-condenser decreases the amount of lead carried by the zinc vapor in the following ways:

1. '.Ihe superheat of the zinc vapor is removed by contact with the molten zinc, the temperature of which does not exceed the boiling point of zinc.

2. The supersaturated lead vapor condenses when brought into intimate contact with the molten zinc.

3. The mechanically entrained droplets of molten lead are rinsed out of the zinc vapor by the molten zinc.

4. The molten zinc tends to dissolve lead vapor so as to establish the decreased vapor tension of lead produced by its solution in zinc.

In the single figure of the accompanying drawing there is diagrammatically illustrated a form of apparatus adapted for the practice of the invention. The apparatus shown in the drawing comprises a redistillation (or spelter) retort 1 of conventional form mounted in an appropriate heating furnace 2 of conventional type. The retort is mounted with a slight forward slope. The front of the retort is in communication with a charging well 3 through which molten zinc metal can be conveniently introduced or fed into the retort. The charging well is equipped with a tap hole 4 on a level with the lowest point of the retort, whereby molten metal enriched in lead may be periodically removed from the retort. Such a charging Well and tap hole make it possible to operate the retort continuously through its entire life. above the charging well serves to convey the zinc vapor volatilized in the retort into a vertical conduit or column 6 having a heat radiating surface of suflicient area and lled with lumps of refractory material 7, such as coke crushed to an appropriate size. flux condenser or fractionating column hereinbefore discussed. A metal casing 8 is spaced from and surrounds the conduit 6. Heat insulating material 9, such as dust coal, is provided in the space between the casing and the conduit, which material may be regulated in amount in order to regulate the dissipation of heat from the column; and thus control its operating tempera- Av cover 10 flts over the column, and is preferably lined with a sealing material 11, such as mud, to prevent the escape of zinc vapor from the system. A zinc vapor conduit 12 leads downwardly from the top of the column to an appropirate condenser 13, or other suitable zinc vapor treatment device. The condenser is equipped with an exhaust outlet 14 and a tap hole 15. A group of such retorts with reflux condensers may be connected in parallel with a sloping zinc vapor conduit and condenser leading into a large collecting pot for molten zinc. Both the conduit and the condenser are advantageously heat-insulated in order to condense zinc vapor in an optimum manner. v

In the redistillation' of zinc in such an apparatus, the temperature in the redux-condenser is The column 6 constitutes the re- A vapor conduit 5 positioned.

automatically maintained at the boiling point oi zinc, provided that the radiating surface of the reflux-condenser is properly proportioned. Thus the' zinc vapor on its way to the condenser traverses a zone inwhich any superheat that it may have acquired is entirely removed and in which it is scrubbed by molten zinc at the boiling point of zinc.

In the practice of the invention in the apparatus hereinbefore described molten zinc metal (of the grade to be refined) is charged from time to time (or continuously if desired) into the retort 1 (through the charging Well 3) to maintain an approximately uniform -volume of molten metal in the retort. A certain proportion of the zinc vapor entering the reflux column is ultimately returned to the retort as molten metal in consequence of the refiuxing action. This refluxed molten metal contains the lead (and other contaminants of higher boiling points than zinc) removed from the zinc vapor, and its return to the retort results in progressively increasing the lead content of the molten metal therein. The retort is continuously operated until the lead content of the molten metaltherein has progressively increased to such a high percentage that the vapor delivered by the reflux column to the condenser 13 has as'high a lead content as can be tolerated. The lead-high molten metal is then removed from the retort and the retortis again charged with molten zinc metal of the grade undergoing purification and the operation is repeated.

Starting with the initial charging of the retort with zinc metal of the grade to be purified. the lead content of the molten metal in the retort gradually increases in consequence of distillation and the refiuxing action, and consequently the lead content of the purified zinc vapor delivered to the condenser 13 correspondingly increases. In other words, `zinc vapor of the highest purity is delivered to the condenser 13 from an initially orfreshly charged retort and the purity of the zinc vapor' delivered to the condenser progressively decreases as the retort continues in operation with periodic charging but without cleaning.

Consequently, the lead content of the molten metal accumulating in the condenser 13 progressively increases, and the operation is interrupted when th lead content of the vapor delivered to the condenser -13 becomes too high for inclusion in the purified zinc product' of the operation. In such a practice of the invention, under commercial operating conditions, zinc metal containing 0.05% lead, or even more, can be economically refined to a lead content of 0.005%, or even less.

The following specific example of the practice of the invention in the apparatus shown in the drawing is illustrative and in no sense restrictive or limiting: The zinc metal to be purified averaged 0.06% lead. Approximately 2400 pounds of metal were volatllized per day of twenty-fourA hours, of which approximately 20% was condensed in the reflux column and returned to the retort as molten metal.r The. retort was` charged at 25 minutel intervals with 30-35 pounds of molten metal of the grade undergoing purification 'to maintain a substantially constant volume of molten metal in the retort. The operation was carried on continuously for ve days. The lead content of the zinc vapor delivered to the condenser 13 at the start ofthe operation was approximately 0.001%,.and at the end of the five days of operation was approximately 0.006% The molten meta-l in the retort at the end of the iive distilling zinc.

days operation contained'approximately 4% lead. The total combined purified zinc metal 'collected in the condenser 13 during the five days of operation contained approximately 0.003% lead.

The preced'ng description has referred more particularly to the purification of impure spelter, hard zinc, dross and the like by redistillation., A similar reflux-condenser may, however, be attached to a retort such as the ordinary horizontal spelter retort producng zinc vapor from ore by reduction. In this case the zinc vapor is admixed with carbon monoxide gas in an amount at least equal to the volume of the zinc vapor as Well as with hydrogen and other gases. In consequence the maximum partial pressure'of zinc vapor in the mixture is one-half atmosphere. For this reason the condensation of zinc in the refluxcondenser does not in this case result in an automatic temperature control as in the case of re- The temperature in the refluxcondenser must be sufiiciently below the boiling point of zinc to condense zinc from gas mixtures conta'ning 50% zinc vapor by volume and less. The temperature in the reflux-condenser in this case is thus a function of the amount of zinc vapor that enters it, so that temperature control is not so simple in this case as in the redist'llation of zinc. It is necessary in this case to adjust the heat radiation from the reflux-condenser to the amount of zinc vapor entering it, for example,-by changing the heat insulation.

While we have hereinbefore described the invention as particularly applied to the purificaton of zinc vapor prior to its condensation to metallic zinc, the purified zinc vapor may be utilized for other purposes, as for example. the production of zinc oxide, zinc dust and other zinc products of high purity. Thus, in the production of zinc' '1. The method of purifying zinc vapor whch comprises scrubbing the vapor with molten zinc ell).

obtained by conde sing part of the zinc vapor and permitting the condensed molten zinc to iiow back into the volatilizing source of the zinc vapor in .counter-current with the znc vapor evolved therefrom. I l

-2. The method of freeing zinc vapor from contaminants with a higher boiling point than zinc which consists in condensing part of the zin'c vapor to molten zinc, returning the condensed molten zinc to the volatilizing source of the zinc vapor "n counter-current with the zinc vapor and `thereby scrubbing the vapor, and conducting the uncondensed portion O f the zinc vapor to an appropriate condenser in which the vapor is condensed to molten zinc.

metal and returned to the retort in co e. The method of. freeing zinc vapor from contaminants with a higher boiling point than zinc which consists in condensing part of the zinc vapor to molten zinc, returning the condensed molten zinc to the volatilizing source of the zinc vapor in counter-current with the zinc vapor and thereby scrubbing the vapor, and conductingthe uncondensed portion of the zinc vapor to an appropriate oxidizing environment whereby the zinc vapor is burned to zinc oxide.

5. The method of freeing zinc vapor from contamnants with a higher boiling point than zinc which consists in condensing part of the zinc vapor to molten zinc, returning the condensed molten zinc to the volatilizing source of the zinc vapor in counter-current with the zinc vapor and thereby scrubbing the vapor, and conducting the uncondensed portion of the zinc vapor to an appropriate condensing environment whereby the zinc vapor is condensed to zinc dust.

6. The method of purifying zinc vapor which comprises passing the zinc vapor through a reduxcondensing columnin which a substantial amount of zinc vapor` condenses and passes out of the column as molten metal invcounter-current ow to the zinc vapor passing therethrough, and condensing the zinc `vapor that passes through the column to molten zinc after the said zinc vapor is passed through the column.

1. The method of purifying `zinc vapor which comprises passing the zinc vapor prior to its final condensation through a fractionating column'of such heat dissipating capacity that a substantial amount of zinc vapor is condensed in the path of the zinc vapor flowingV through the column and passes out of the column as molten 'metal in4 counter-current flow to the vapor stream, and condensing the zinc vapor that passes through the column to molten zinc after the said zinc vapor is passed through the column. ,l

8. The method of redistilling zinc which comprises volatilizng the zinc in a retort, conducting the zinc vapor into a reflux condenser where a portion of the zinc vapor is condensed molten ter-current flow with the zinc vapor, and conducting the zinc vapor that passes through the reflux condenser to an appropriate condenser in which it is condensed to molten'zinc. Y

9. 'I'he method of redistilling zinc which comprises volatilizing the zinc in a retort, conducting the zinc vapor into av reux condenser where a portionof the zinc vapor is condensed to molten metal and returned to the retort in countercurrent ow with the zinc vapor, the'ow of both the condensed molten metal and the zinc vapor through the reflux cigoegilsfr being along a tortuous path, and ap lately condensing to molten metal the zinc vapor that passes through the reflux condenser. g

l 10. 'I'he method of purifying metallic zinc by redistillation which comprises vaporizing the zinc, passing the zinc vapor through a reflux condensing column in which a substantial amount of zinc vapor condenses and passes out of the column in counter-current ow to the zinc vapor passing therethrough, and appropriately condensing the zinc vapor that passes through the column to through the column.

11. Themethod of purifying metallic zinc by redistillation which ycomprises lpassing the zinc vapor prior to condensation through a fractionating column of such heat dissipatinglcapacity that a substantial amount of zinc vapor is conmolten Vzinc after the said zinclvapor is passed densed in the path of the stream of vapor flowingk through the column and passes out of the column in counter-current flow to the. vapor stream, and condensing the zinc vaporthat,

passes through the column to molten zinc after the said zinc Vapor is passed through the column. A 12. In a method of purifying zinc vapor, the

step which comprises bringing zinc vapor con` ing to claim 12, in which the zinc vapor is passed upwardly in contact with molten zinc passed downwardly.

14. In a 'method of purifying lead-contaminated zinc vapor coming fromthe reduction of zinciferous material, the step which comprises bringing a mixture of the zinc vapor and reduction gases into intimate contact with molten zinc whereby a substantial amount of the lead is retained by the molten zinc while substantiall lead-free zinc vapor passes on. V y

15. A method of purifying lead-contaminated zinc vapor coming from the reduction of zinciferous materials according to claim 14, in which the zinc vapor and reduction gases are passed upwardly in contact with molten zinc passed downwardly. 1

16. A method of purifying zinc vapor contaminated with an objectionable amount of lead, which comprises scrubbing the lead contaminated zinc vapor with molten zinc until the lead is abstracted from tlie zinc vapor by the molten zinc.

1'7. A method of purifying zinc vapor contaminated with an ,objectionable amount of lead,

is abstracted from the' zinc vapor by the molten zinc, and permittingthe de-leaded zinc vapor to,

pass on.

18. The method of purifying zinc by distinstion lwhich comprises volatilizing the zinc, passing the resulting vapor upwardly through a reflux condensing column and therein condensing` a substantial portion of the ascendingv vapor while permitting a substantially larger portion of the ascending vapor to reach the top of the column, permitting a substantial amount of molten metal resulting from he condensation of ascending vapor. to flow out f the column in countercurrentA contact with the ascending vapor, and recovering the vapor reaching the top of the column.

19.V The method of purifying Izinc lby distillation which comprises volatilizing the zinc, passing the resulting vapor upwardly in a ltortuous path through a reflux condensing column and therein condensing a substantial portion of the ascending vapor while 'permitting a substantially larger portion of the lascending vapor to reach the top of the column, permitting a substantial amount of molten metal resultingy from the condensation 4of ascendingvapor to flow out of the column in counter-current contact with the une' by dismation which comprises volatilizingth zinc, pass--v ing the resulting vapor upwardly through 'a reflux condensing column and thereincondensing a substantial portion of the ascending vaporI while permitting a. substantially larger portion of the ascending .vapor to reach the top of the co1umn,7permtting a substantial amount of molten metal resulting from the condensation of ascending vapor to ow out of the column in counter-current contact with the ascending vapor and to return to the zinc undergoing vo1atilization, and condensing to moltenmetal the vapor reaching the top of the column.

LEON S. HOLSTEIN.` PHILIP M. GINDER. 

